MDEV-4422 SHOW PROCESSLIST reference to THD::db not protected against simultaneous updates

protect THD::db with THD::LOCK_thd_data

sql/sql_class.h

@@ -1605,6 +1605,7 @@ public:
     Protects THD data accessed from other threads:
     - thd->query and thd->query_length (used by SHOW ENGINE
       INNODB STATUS and SHOW PROCESSLIST
+    - thd->db and thd->db_length (used in SHOW PROCESSLIST)
     - thd->mysys_var (used by KILL statement and shutdown).
     Is locked when THD is deleted.
   */
@@ -2838,6 +2839,7 @@ public:
   */
   bool set_db(const char *new_db, size_t new_db_len)
   {
+    mysql_mutex_lock(&LOCK_thd_data);
     /* Do not reallocate memory if current chunk is big enough. */
     if (db && new_db && db_length >= new_db_len)
       memcpy(db, new_db, new_db_len+1);
@@ -2850,6 +2852,7 @@ public:
         db= NULL;
     }
     db_length= db ? new_db_len : 0;
+    mysql_mutex_unlock(&LOCK_thd_data);
     return new_db && !db;
   }
 
@@ -2866,8 +2869,13 @@ public:
   */
   void reset_db(char *new_db, size_t new_db_len)
   {
+    if (new_db != db || new_db_len != db_length)
+    {
+      mysql_mutex_lock(&LOCK_thd_data);
       db= new_db;
       db_length= new_db_len;
+      mysql_mutex_unlock(&LOCK_thd_data);
+    }
   }
   /*
     Copy the current database to the argument. Use the current arena to

sql/sql_show.cc

@@ -2212,10 +2212,10 @@ void mysqld_list_processes(THD *thd,const char *user, bool verbose)
 	  thd_info->host= thd->strdup(tmp_sctx->host_or_ip[0] ?
                                       tmp_sctx->host_or_ip :
                                       tmp_sctx->host ? tmp_sctx->host : "");
-        if ((thd_info->db=tmp->db))             // Safe test
-          thd_info->db=thd->strdup(thd_info->db);
         thd_info->command=(int) tmp->command;
         mysql_mutex_lock(&tmp->LOCK_thd_data);
+        if ((thd_info->db= tmp->db))             // Safe test
+          thd_info->db= thd->strdup(thd_info->db);
         if ((mysys_var= tmp->mysys_var))
           mysql_mutex_lock(&mysys_var->mutex);
         thd_info->proc_info= (char*) (tmp->killed >= KILL_QUERY ?